Spiral launders

ABSTRACT

When flowing molten metal down a spiral channel within an evacuated vessel to effect rapid distillation of, say, zinc from zinc-contaminated molten lead, splashing occurs and much lead is carried over with the zinc distillate. To overcome this, brakes in the form of upstanding ridges running along the spiral path within the spiral channel are provided to increase the wetted perimeter of the channel which maintains the internal turbulence of the molten metal and also renders the surface thereof adequately smooth to minimize splashing. When the molten metal is zinc-contaminated lead, it is necessary that the speed be no greater than 5 ft./sec.

United States Patent Inventors Leon Stocks;

John Frederick Castle, both of Avonmouth, England Appl. No. 883,657 Filed Dec. 9, 1969 Patented Oct. 19, 1971 Assignee The Broken Hill Associated Smelters Proprietary Limited Melbourne, Australia Continuation-impart of application Ser. No. 661,397, Aug. 17, 1967, now abandoned.

SPIRAL LAUNDERS 6 Claims, 1 Drawing Fig.

U.S. Cl 266/34 V, 75/14, 75/78, 202/175, 202/205, 266/37 Int. Cl C22b 9/04 Field of Search 266/37; 202/175, 205

Primary Examiner-Gerald A. Dost Attorney-Holman & Stern ABSTRACT: When flowing molten metal down a spiral channel within an evacuated vessel to effect rapid distillation of, say, zinc from zinc-contaminated molten lead, splashing occurs and much lead is carried over with the zinc distillate. To overcome this, brakes in the form of upstanding ridges running along the spiral path within the spiral channel are provided to increase the wetted perimeter of the channel which maintains the internal turbulence of the molten metal and also renders the surface thereof adequately smooth to minimize splashing. When the molten metal is zinc-contaminated lead, it is necessary that the speed be no greater than 5 ftJsec.

Pmmennm 19 m1 SPIRAL LAUNDERS This application is a continuation-in-part of U.S. Pat. application Ser. No. 661,397, filed Aug. 17, 1967 and now abandoned.

This invention relates to spiral launders and has application in apparatus used for the distillation of metals under vacuum. It has particular application in apparatus used in removing zinc from zinc-contaminated molten lead under vacuum.

It is known to flow molten zinc-containing lead down a spiral channel within an evacuated vessel and to evaporate the zinc from the molten contaminated lead flowing down the channel. The zinc vapor is then condensed on a central core about which the spiral channel is disposed. Such zinc vapor condensation can be seen, for example, in Us. Pat. No. 3,220,827. However, this patent does not utilize spiral channel launders, but rather concerns itself with an even tangential flow of zinc-contaminated lead over the upper edge of an inverted frustoconical tube and then down its surface which forms an evaporation zone.

The instant spiral channel launder takes the form of spiral channel flights about a central zinc vapor condensing core. The width to pitch relationship of the spiral channel has to be a compromise giving adequate distillation area (the evaporation zone or distillation area is the surface area of the spiral channel) on one hand, yet allowing enough space between the flights of the spiral channel to permit easy access and cleaning. Furthermore, the surface of the zinc-contaminated lead traversing the spiral channels must be maintained smooth so splashing does not readily occur, yet internal turbulence is necessary to circulate the contaminated lead within the channel to continually expose different quantities thereof to the distillation area.

It is one of the objects of this invention to overcome this tendency to splash and thereby decrease the less volatile metal carry over to the condensed volatile metal on the central core.

It is another object of this invention to provide a braking means in the channel to maintain the molten metal surface in the channel sufficiently smooth, yet causing internal turbulence.

The invention includes a process for distilling a more volatile molten metal from a less volatile molten metal wherein the wetted perimeter of the spiral channel launder is increased at a far greater rate than the rate of change of the cross-sectional area of flow. Since both the hydraulic radius and linear velocity are reduced, the internal turbulence of the molten metal maintained and its surface remains adequately smooth. The internal turbulence should be in the order of at least 3,000 Reynolds number.

The invention further includes an apparatus wherein the flight of the spiral channel is a shelf substantially horizontal in radial cross section bounded by inner and outer raised edges and having at least one raised ridge or brake extending along the inner surface of the channel in the direction of the spiral to brake the natural rate of descent of the molten metal. By means of this braking means the linear flow rate is reduced, and in the case of zinc-contaminated lead, it is reduced to a speed less than ft./sec. which is a critical maximum velocity to prevent intolerable splashing.

Preferably the spiral channel is attached to a cylindrical or frustoconical backing plate, i.e. it runs around the inner surface of a hollow cylinder or inverted truncated cone.

The inner raised edge is preferably inclined inwardly toward the axis of the spiral.

The outer edge may be a continuous outer cylindrical or frustoconical wall.

The invention will be further described with reference to the accompanying drawing which is a cross section of a flight of the channel of a spiral launder.

In the drawing the wall 1 of a hollow cylinder or an inverted truncated cone of metal sheet has attached to it a spiral 2 in the form of a horizontal shelf with an inner raised edge 5.

The base of the shelf is formed with two parallel ridges 3 of rectangular section extending in the direction of the spiral.

The surface of the flowing metal-is indicated by the reference 4.

The ridges, as shown, act as brakes by increasing the wetted perimeter of the channel, thus giving a slower average velocity and reducing the danger of splashing due to'the breaking of the metal surfaceby solid accretions, imperfections in the spiral, etc.

In the case of distilling zinc from 'zinc c'on'taminated lead and when the wetted perimeter 'of the spiral channel is increased by about 40 percent by use of ridges 3, the following results are obtained. It should be noted that since the distillation rate and purity of the lead are markedly dependent on the temperature of the molten lead/zinc, this table shows generally the type of results in product purity which can be obtained. ln the last'analysis, the mean lead temperature at the inlet is governed by the requirements of efficient condenser operation from which the molten lead/zinc is obtained.

429 ton/hr. 236.6 ton/hr.

It will be seen that the lower mean lead temperature for the braked system is a slight disadvantage inasmuch as a lower mean lead temperature results in a reduction in the zinc distillation rate. This could mean a consequent reduction in the distillate purity levels if it were not for the fact thatlead contamination in the zinc distillate is unexpectedly reduced.

It will be seen that the lead in the zinc distillateis decreased from 0.100 percent (1) to 0.0436 percent (ll), some 67 percent.

Various modifications may be made within the scope of the invention. Thus, the ridges may be of other cross section than rectangular and some element of sideways displacement of the ridges in relation to the path of the shelf, i.e. towards the inner edge may be incorporated to resist the centrifugal force ex- 'erted on the molten metal.

We claim:

1. A spiral launderer for separating a volatile molten metal constituent from a relatively less volatile molten metal constituent by flowing molten metal containing both constituents down a spiral channel within an evacuated vessel to effect distillation of the volatile metal and form a highly pure distillate, comprising a housing structure having attached thereto a spiral channel, the bottom of which is substantially horizontal in radial cross section and is bounded by inner and outer walls, said channel bottom having at least one raised ridge running in the direction of the spiral path of said channel to brake the natural rate of descent of the molten metal and produce turbulence in the body of the molten metal while the surface thereof remains sufficiently smooth so as to minimize splashing.

2. A spiral launderer as claimed in claim 1 in which the housing to which said spiral channel is attached is a cylinder, the spiral channel being attached to the inner wall thereof, and said cylinder forming the outer wall bounding the spiral channel bottom.

3. A spiral launderer as claimed in claim 1 in which said housing to which the spiral channel is attached is a frustoconical truncated cone, the spiral channel is attached to the inner surface thereof, and the inner surface forms said outer wall bounding the bottom of said spiral channel.

' 3,614,081 3 4 4. A spiral launderer as claimed in claim 1 in which said braked rate of said molten metal is no greater than S ft./sec.

inner wall of the spiral channel is inclined inwardly towards 6 A Spi al laundef r as claimed in Claim 1 i iw i h r the axis of the spiral. are two raised ridges having rectangular cross section on the 5. A spiral launderer as claimed in claim 1 in which the bottom s i pir l Chann l- 

1. A spiral launderer for separating a volatile molten metal constituent From a relatively less volatile molten metal constituent by flowing molten metal containing both constituents down a spiral channel within an evacuated vessel to effect distillation of the volatile metal and form a highly pure distillate, comprising a housing structure having attached thereto a spiral channel, the bottom of which is substantially horizontal in radial cross section and is bounded by inner and outer walls, said channel bottom having at least one raised ridge running in the direction of the spiral path of said channel to brake the natural rate of descent of the molten metal and produce turbulence in the body of the molten metal while the surface thereof remains sufficiently smooth so as to minimize splashing.
 2. A spiral launderer as claimed in claim 1 in which the housing to which said spiral channel is attached is a cylinder, the spiral channel being attached to the inner wall thereof, and said cylinder forming the outer wall bounding the spiral channel bottom.
 3. A spiral launderer as claimed in claim 1 in which said housing to which the spiral channel is attached is a frustoconical truncated cone, the spiral channel is attached to the inner surface thereof, and the inner surface forms said outer wall bounding the bottom of said spiral channel.
 4. A spiral launderer as claimed in claim 1 in which said inner wall of the spiral channel is inclined inwardly towards the axis of the spiral.
 5. A spiral launderer as claimed in claim 1 in which the braked rate of said molten metal is no greater than 5 ft./sec.
 6. A spiral launderer as claimed in claim 1 in which there are two raised ridges having rectangular cross section on the bottom of said spiral channel. 